corrected Zimbra URL, fixes #222

README.md

@@ -189,7 +189,7 @@
   * [Horde Groupware](http://www.horde.org/apps/groupware) - PHP based collaborative software suite that includes email, calendars, wikis, time tracking and file management.
   * [Kolab](https://kolab.org/) - Another groupware suite.
   * [SOGo](http://www.sogo.nu/) - Collaborative software server with a focus on simplicity and scalability.
-  * [Zimbra](http://community.zimbra.com/) - Collaborative software suite, that includes an email server and web client.
+  * [Zimbra](https://www.zimbra.com/open-source-email-overview/) - Collaborative software suite, that includes an email server and web client.
 
 
 ## Configuration Management Database